#2 - Balancing Redox
Reactions Using the
Oxidation Number Method:
Balance the following redox equation using either the
inspection technique or the oxidation number method. Be sure to
check that the atoms and the charge are balanced.
Cu(s) + HNO3(aq)
+ NO(g) + H2O(l)
The nitrogen atoms and the oxygen atoms are difficult to
balance by inspection, so we will go to Step #3. The copper atoms are changing
their oxidation number from 0 to +2, and some of the nitrogen atoms are changing
from +5 to +2. These changes indicate that this reaction is a redox reaction. We
next determine the changes in oxidation number for the atoms oxidized and
0 to +2 Net Change = +2
+2 Net Change = -3
We need three Cu atoms (net change of +6) for every 2
nitrogen atoms that change (net change of -6). Although the numbers for the
ratio determined in Step #5 are usually put in front of reactant formulas, this
equation is somewhat different. Because
some of the nitrogen atoms are changing and some are not, we need to be careful
to put the 2 in front of a formula in which all of the nitrogen atoms are
changing or have changed. We therefore place the 2 in front of the NO(g) on the
product side. The 3 for the copper atoms can be placed in front of the Cu(s).
3Cu(s) + HNO3(aq)
--> Cu(NO3)2(aq) + 2NO(g) + H2O(l)
We balance the rest of the atoms using the technique
described in Chapter 4, being careful to keep the ratio of Cu to NO 3:2.
3Cu(s) + 8HNO3(aq)
3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
here to see an exercise that will allow you to try this task yourself.
Return to the
Balancing Redox Equations Page.